1. Field of the Invention
This invention relates to an automatic driving speed control system and method for a motor vehicle, more particularly to a method and apparatus by which an optionally determined fixed vehicle speed can be maintained automatically and which automatically controls the distance from a motor vehicle driving ahead.
2. Description of the Prior Art
Fixed speed driving devices are used to control the speed of a motor vehicle into which such a device has been fitted. When the speed of a motor vehicle is at a certain desired rate, the fixed speed driving device automatically controls the speed to hold it constant at that rate, without need for the driver to touch the accelerator. This reduces driver fatigue, especially when driving continuously at a fixed speed on highways.
Fixed speed driving, however requires braking when the motor vehicle driving ahead slows down. Braking releases the automatic fixed speed driving control, and after each braking action it is necessary to reset the device. This makes practical, proper function of the fixed speed driving device difficult in heavy traffic.
Improved fixed speed driving devices are known in the prior art and are described, for example, in Japanese Public Disclosure No. 52(1977)-110388, which relates to a fixed speed driving device further provided with a vehicle interval control device which automatically controls the speed of the motor vehicle to a constant speed when the vehicle interval is great and controls the vehicle speed to maintain the optimum vehicle interval when the interval is short. This device automatically adjusts the speed of the vehicle such that the driver almost never needs to control the driving speed thus giving much improved heavy-traffic operation.
The above-mentioned improved prior art device, however, leaves one particular, practical problem unsolved. Ordinary fixed speed driving systems use Doppler radars or the like to detect the distance between the device-fitted vehicle and the vehicle ahead. Thus when a curve or bend in the road is encountered, the Doppler beam loses the motor vehicle in front as it enters the curve. It therefore appears to the device that the distance between the motor vehicles has increased greatly, and the device then accelerates the vehicle to the preset speed. This may require the operator to override the control system due to the decreasing real distance between the vehicles.